Transience

Greater North America, The Combine, UCAS, USoNC, The Union,

By many names and many iterations it's common in Science Fiction to have the USA annex Canada in the least through the western hemisphere completely. Never mind that annexing even Canada would be prohibitively expensive to synchronize. My theory is USA size is increased to be representative of how big the country looms in the imagination and to show how much more powerful this future form is. It also allows for narrowing the players in the story. Major nations such as Brazil, Venezuela, Columbia and Argentina can be safely ignored despite the sizable power they represent, and greater potential, on the world stage. On the other hand these worlds usually have the EU as a unified nation and often a pan-African or Arabian Superstate as well so it may just be an attempt to keep the USA up to speed with it's rivals.